During the last decades, optical discs have become an important data storage medium. Optical discs can be classified into different categories: read-only discs, write-once discs and re-writable discs. The read-only discs only allow to read data stored on the discs, whereas the write-once discs additionally allow to record data. While the data recorded onto write-once discs cannot be deleted in order to record new data, re-writable discs have the possibility to do so.
FIG. 1 shows the arrangement of a known write-once (in the following also referred to as “recordable optical disc”) disc 1 exemplified by a CD-R disc. The disc 1 comprises a transparent substrate (for example polycarbonate) layer 2, a recording layer 3, a reflective layer 4 and a protective layer 5. The substrate layer 2 is provided with grooves 6 which are extending along the spiral shaped data storage path which is shown in FIG. 2.
In order to record information onto the disc 1, a recording laser beam 7 irradiates the recording layer 3 through the substrate layer 2, thereby forming “pits” 8 (writing marks), i.e. areas of the recording layer 3 showing a different reflection behavior than the rest of the recording layer 3 positioned within/above the grooves 6. The length of the pits 8 and the distance between the pits (in the following referred to as lands) along the groove which is extending along the spiral shaped data storage path represents a sequence of binary “0”-values and “1”-values which itself represent the data stored on the disc 1.
FIG. 3 shows the arrangement of a known read-only disc 1′. The disc 1′ comprises a transparent substrate layer 9, a reflective layer 10 and a protective layer 11. The read-only disc 1′ contains a spiral shaped data storage path of successive shallow depressions 8, also called pits, which are ‘pressed’ into the substrate layer 9 by a stamper during a replication process forming an information layer 9′. This information layer 9′ is coated by a reflective layer 10. Encoded information is stored in the length of the pits 8 and in the distances between the pits (in the following also referred to as lands 8′). During playback of the read-only disc 1′ a scanning-like spot 12 which is focused by an objective lens 13 is diffracted by the pits 11 in the information layer 9′. Additionally there is some destructive interference between the laser light reflected from the pits and the laser light reflected from the area around the pits. The laser light that is reflected and diffracted back into the objective lens 13 is modulated according to the information encoded within the information layer 9′. The intensity of the laser light that is reflected and diffracted back into the objective lens 13 is converted into an electrical signal also called high frequency (HF) signal. This HF-signal is pre-amplified, filtered, equalized and converted into a digital signal (in case of a compact disc this signal is called EFM-signal) by an auto-slicer which extracts the positions of the crossings of the HF-signal with a decision level. The resulting digital signal is then decoded, error-corrected and fed into the further signal processing units. The read out process of the write-once disc 1 is performed in a similar way.
A problem which may occur over a period of time, e.g. years, is that the chemical composition or the physical properties of the recording layer 3 or of the reflective layer 4 of the disc 1 may change in a way that information stored within the recording layer 3 may no longer be read out. For example, in case of a recordable layer based on an organic dye layer the chemical composition of the recording layer 3 may change such that the refractive index of the recording layer at the location of the pits 8 coincides with or is close to the refractive index of the rest of the recording layer, thereby making it impossible to read the information originally stored onto the disc 1. In a similar way, problems may occur if the reflective layer 10 of the read-only disc 1′ shown in FIG. 3 or the reflective layer 4 of the write-once disc 1 shown in FIG. 1 deteriorates. In this case, information originally stored on the read-only disc 1′ or on the write-once disc 1 will no longer be readable.
It is an objective of the present invention to provide a method for restoring data stored on optical discs if a normal read out of said data is no longer possible due to deterioration effects as described above.